52 



MENDELISM 



CHAP. 



of this case is that all the animals are black, and that 

 we are dealing with the presence and absence of two 



factors, a colour devel- 

 oper (C) 3 and a colour 

 modifier (G), both act- 

 ing, as it were, upon 

 a substratum of black. 

 The F 2 generation 

 really consists of the 

 four classes agoutis, 

 blacks, albino agoutis, 

 and albino blacks in 

 the ratio 9:3:3:1. 

 But since in the 

 absence of the colour 



Diagram to illustrate the nature of the F 2 generation 



which may arise from the mating of agouti with developer C the Colour 

 albino in mice or rabbits. 



modifier G can pro- 

 duce no visible result, the last two classes of the ratio are 

 indistinguishable, and our F 2 generation comes to consist 

 of three classes in the ratio 9:3:4, instead of four classes 

 in the ratio 9:3:3:1. 



This explanation was further tested by experiments 

 with the albinos. In an F 2 family of this nature there 

 ought to be three kinds, viz. albinos homozygous for G 

 (GGccBB), albinos heterozygous for G (GgccBB), and 

 albinos without G (ggccBB). These albinos are, as it 

 were, like photographic plates exposed but undeveloped. 



GCB.'-.'v.'v. 

 GCB/-.Y-.Y- 



gcb::::'. 



GcB :■;■;•:• ;■• 



GCB'/.v.v 



gCB;';:;;; 



GCB-:::: 



gcB-^//: 



•>.' •"Agouti'- 



.■•.'•'Agouti': 



.'-.'Agouti'. 



\\ Agouti'-. 



GcB 

 GCB'/ :':':': 



GcbHB 



Gc3| 



GcB'-.-.'v.'v 



.'■.'•:^?. uti: 



GcB ■ 



^cBi 



X; Agouti; ( 



BLACK 



rCB/.v.v. 



*CB ;■■/■/•/: 



gCB: ;:•';■;• 



GcB"::;::: 



gCB 

 gCB 



Albino 



gCB 

 gcB 



Albino 



'■'Agouti'. 



.'•.'•.Agouti! 



JcB.-.v/.v! 



GCB'.v.v'; 



gcBl 

 GcBl 



gcB 

 gCB 



Albino 



gcB 

 gcB 



Albino 



Agouti: 



■3HS! 



Fig. 3. 



